There are many systems and known methods for discharging solids from a fluidized bed pressure vessel, gas phase fluidized bed pressure vessel, or gas phase fluidized bed polymerization vessel. However, the use of existing discharge systems and methods can result in excess loss of reactant from the discharge system. Specifically, a significant amount of the gas or gas/liquid mixture is lost because the void space within and around the particles is filled with the high pressure gas mixture. The lost gas must then be either replaced, consuming additional raw materials, or recycled back into the system via compression, condensation with pumping, or a combination of these. In either scenario, raw materials are wasted and energy consumed.
One process that involves the discharge of a gas/solids mixture from a pressure vessel is the process for the manufacture of polyolefin resins, thereby involving the polymerization of olefin monomers in a fluidized bed reactor. An example of a process for the manufacture of polyolefin resins is disclosed in U.S. Pat. No. 4,003,712 (“the '712 patent”). As therein defined, a product is discharged from the reaction zone through a gas lock zone and the unreacted monomer that accompanies the resin is vented and recycled back to the reaction zone by compression. The product is then transferred to downstream equipment via a conventional dilute phase conveying system.
An alternative discharge system is described in U.S. Pat. No. 4,621,952 (“the '952 patent”). Referring to FIG. 1, a prior art gas lock zone system involving multiple settling vessels operating in series, is shown. The '952 patent describes that the gas mixture lost from the process could be significantly reduced by using the gas displacing ability of solids using two or more vessels with pressure equalization between each. As practiced today, a valve 10 between a nozzle on the fluidized bed pressure vessel 1 and settling vessel 4 is opened, and solids along with pressurized gas enter settling vessel 4. A second connection 9 between the top of settling vessel 4 to a slightly lower pressure section of the reactor provides a flow path for the gas while solids settle out to essentially fill settling vessel 4. Both valves 10 and 9 are then closed, leaving settling vessel 4 full of the solid particles, but with interstitial spaces between the particles filled with the gas mixture, and settling vessel 4 at full reactor pressure.
A valve (shown but not numbered) is then opened and solids are transferred to a transfer tank 13. As the solids flow into transfer tank 13, pressure equalization also occurs between transfer tank 13 and product chamber tank 4. Upon completion, the pressure in transfer tank 13 and product chamber tank 4 is less than the reactor pressure and the product may be transferred to other vessels for additional processing with only a modest pressurized gas transfer therein.
Once empty, the role of each vessel changes to the gas receiver function described above. The gas received by the settling vessel 4 is then transferred back into the fluidized bed pressure vessel 1 during the next fill cycle. While more effective than the process described by the '712 patent, the '952 patent has some drawbacks. Initially, the fluidized bed pressure vessel must be elevated because the settling vessels and product tanks are stacked and located below the fluidized bed. Furthermore, the transference of the solid from the settling vessels to the product tanks requires time, therein limiting the number of discharge cycles possible in a given period (typically to 20 to 30 discharges per hour). Moreover, because the tanks work in series, if any one tank in a series pair is taken out of service for cleaning or maintenance, the entire series is inoperative, and the gas losses from the other series in a series pair is increased.
U.S. Pat. Nos. 6,255,411 and 6,498,220 describe improvements to the gas lock concept using two parallel sets of vessels with two or three series vessels per set. This prior art discharge system has multiple pressure equalization steps to improve efficiency. In such an arrangement if one tank must be removed from service for cleaning it disables all tanks in a vertical set, but allows continued operation with the other parallel set. However, capacity is essentially cut in half and recovery efficiency is reduced as the cross set equalizations are not available. The '411 patent offers a faster cycle time, but during some steps, there is only one closed valve to prevent blow-by of pressurized gases to downstream equipment.
Other background references include U.S. Pat. No. 6,472,483, EP 0 250 169 A, and WO 2006/079774.
Accordingly, there exists a need for a method to remove solids from a fluidized bed pressure vessel with a reduced loss of gas and reactants, and wherein the discharge system allows, for example, for at least one of a higher discharge capacity, less downtime due to maintenance, greater efficiency in the processing of solids, and improved safety.